Estimate of Variations in the Parameters of the Midlatitude Lower Ionosphere Caused by the Solar Flare of September 10, 2017

Abstract

Changes in the state of the D and E ionospheric regions lead to variations in the amplitude-phase characteristics of VLF radio signals. The existing theoretical and empirical models of the propagation of low-frequency electromagnetic waves qualitatively describe the relative variations in the parameters of the lower ionosphere associated with strong heliogeophysical disturbances; however, these models do not allow estimation of the absolute value and distribution of the electron concentration. We used the measurement data for the amplitude-phase characteristics of VLF radio signals with different frequencies propagating along two closely spaced paths. This made it possible not only to quantify the parameters of the D region of the ionosphere on a spatial scale of thousands of kilometers during the powerful solar flare of September 10, 2017 but also to restore the electron concentration profile before the onset of X-ray radiation.